, 1996, Kanter and Fordyce, 1993 and Watchko et al., 1988). Findings in these studies raise the possibility

that some central (Gandevia, 2001) or local (Parthasarathy et al., 2007) mechanism may inhibit the respiratory muscles in the face of increased mechanical loads, and thus protect them against fatigue and damage – although at the cost of carbon dioxide (CO2) retention. Experimental evidence supports the existence of local protective mechanisms (Laghi et al., 2003, Mador et al., 1996 and Eastwood et al., 1994). In patients who developed hypercapnia during a failed trial of weaning from mechanical ventilation, we observed sequential recruitment of the extradiaphragmatic muscles (Parthasarathy et al., 2007). The sequence began with greater-than-normal activity of inspiratory muscles followed by expiratory

muscle recruitment. It is known that expiratory muscle activity is not confined to exhalation, but can also occur during inhalation selleck chemical and thus limit inspiratory shortening of the diaphragm (Abe et al., 1999). As such, recruitment of extradiaphragmatic muscles may have a dual role during loading: to protect the diaphragm against contractile fatigue, and to improve diaphragmatic neuromechanical coupling by limiting diaphragmatic shortening. Evidence also supports the existence of reflex mechanisms that inhibit central neural output under loaded conditions. Implicated mechanisms include group III and IV afferents and mechanoreceptors originating in the contracting respiratory

muscles (Gandevia, 2001). Reflex inhibition of central neural output causes hypercapnia, a potent source of air hunger (Banzett et al., 1996). This selleck consideration raises the possibility that reflex inhibition of central neural output during loading may also have a dual role: to protect the respiratory muscles against damage and contractile fatigue, and to trigger intolerable air hunger, leading to task failure. The objective of the current study, conducted in healthy volunteers, was to elucidate the physiological mechanisms involved in the development of CO2 retention during progressive inspiratory threshold loading. Amino acid In subjects undergoing progressive inspiratory threshold loading, we hypothesized that improvements in diaphragmatic neuromechanical coupling secondary to extradiaphragmatic muscle recruitment are insufficient to prevent alveolar hypoventilation and task failure, and the latter will result primarily from reflex inhibition of central neural output to the diaphragm and air hunger rather than contractile fatigue. Experiments were performed on 18 healthy subjects (4 female), mean (±SE) age 33 ± 2 years; all but one were naïve to the investigation’s purpose. The study was approved by the Institutional Review Board of Edward Hines, Jr. Veterans Affairs Hospital, which conforms to the provisions of the Declaration of Helsinki. Informed consent was obtained in writing from all subjects. Measurements.